The preparation of the test pieces is an extremely important part of the testing itself and it must not be the case that the test results reflect the effects of the preparation rather than the properties of the materials being tested.
Related Standards
ISO 4661, ISO 23529
Introduction
The testing of rubber is either starting from uncured rubber or from cured rubber products. Testing can also apply to the uncured or cured state. The preparation of the test pieces is an extremely important part of the testing itself and it must not be the case that the test results reflect the effects of the preparation rather than the properties of the materials being tested.
Temperature conditioning of the test material is also an important part of the preparation.
Testing cured properties – ISO 4661
In order to test cured properties, specimen test sheet and other test pieces need to be produced. Nearly all rubber testing is carried out on test pieces punched from 2 mm ± 0,2 mm thick test sheets, which are vulcanized in a mould, often with four cavities, each 150 × 150 mm.
Cylindrical test pieces used for compression set, relaxation, abrasion and compression testing are also produced in a mould. There are two common sizes, the small test piece with a diameter of 13 mm and a height of 6,3 mm, and a large test piece 29 mm in diameter and 12,5 mm high.
Test pieces are punched out of the test sheets to perform tests for tensile strength, tear strength and tension set etc. See figure 1.
It is important that sharp punching cutters or dies are used against a soft surface when punching out the test specimens. Punching cutters with poor cutting edges or nicks may cause rupture notches in the test piece and thus produce incorrect results.
Testing uncured properties
When testing uncured rubber, it is often sufficient to cut off a piece of rubber of the appropriate size using a pair of scissors. In certain tests, as for example, the determination of cure curves, the test must have a specific volume in order to be able to provide reproducible test results. In these cases, a volume press is used to provide test pieces with a constant volume.
See figure 2.
Testing materials in products
In certain cases, one is interested in testing the material properties of a rubber product. Sheets 2 mm thick or cylindrical pieces are cut from the product for testing. A slicing machine (see figure 3) or a cutting machine is very useful for this purpose and consists of a band knife or a rotating blade, which can cut off thin strips from a product.
The cylindrical pieces are cut from thicker parts of a product with a rotating knife.
See figure 4.
Conditioning ISO 23529
Prior to testing, the test material must often be conditioned. After test sheets are cured, there is a specified waiting time of at least 16 hours before testing, in order to allow the material to stabilize. Test pieces must also be given time to assume the testing temperature, which may take a few hours. Certain materials such as cellular materials must also be conditioned to achieve the correct balance of moisture, which may take a considerably long time. The standardized climatic conditions in a laboratory are 23 °C ± 2 °C and 50 % RH ± 5 % RH.
Conditioning time for rubber
Time in minutes to obtain evenness of temperature according to ISO 23529.
| Thickness, mm | Temperature, °C | Time, minutes |
| 25 | -50 ± 0 100 |
135 95 140 |
| 10 | -50 ± 0 100 |
45 30 45 |
| 5 | -50 ± 0 100 |
20 15 20 |
Read more about Elastocon's equipment for sample preparation here
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And for further reading, please find the referenced standards at:
ISO 4661-2:2018: https://www.iso.org/standard/74945.html
ISO 23529:2016: https://www.iso.org/standard/70323.html
Göran Spetz
